Television transmitter modulator



Patented Nov. 11, 1941 TELEVISION TRANSMITTER MoDULAToa Thomas L.Gottier, Westmont, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application July 21, 1939, Serial No. 285,666

(oli. 179-171) 7 Claims.

This invention relates to television transmitter modulators and, moreparticularly, to a modulator which covers a wide frequency band withincreasing gain at the higher frequencies.

In television transmitters of the present day type, a modulation band ofseveral million cycles is required to transmit pictures of the desireddetail. The frequency band covered by a conventional modulator generallyexhibits a gain characteristic which falls off at the higher fre-`quencies and, furthermore, the phase may be shifted by an undesirableamount. While the attenuation may be corrected at the higherfrequencies, the correcting means often introduces more than thepermissible phase shift with resulting` picture distortion.

It is one of the objects of the present invention to provide means formodulating a television transmitter. Another object is to provide meansfor modulating a carrier over a frequency band of several millioncycles. Another object is to provide a television modulator with meansfor obtaining increasing gain as the frequency increases Withoutshifting the phase beyond permissible limits.

The invention will be described by referring to the accompanying drawingin which Figure 1 is a circuit diagram representing one embodiment ofthe invention; Figures 2 and 3 are circuit diagrams of modifications ofthe invention; Figures 4 and 5 are graphs used in describing theinvention. In the several figures, similar reference numerals will beused to indicate similar elements.

In modulators of the character about to be described, thermionic tubesare employed. The input impedance of such tubes depends upon theiroutput load. This is especially noticeable at the higher operatingfrequencies. Since the tubes have capacity between their input elements,the input reactance varies with frequency. Furthermore, the effectiveinput resistance is not consta'nt with frequency if the input is coupledto the output load as is the usual case.

When the modulator covers a narrow frequency band, the effect of theinput characteristic may be neglected; for a Wide band, the varyingimpedance has a detrimental effect. This effect cannot be neutralized bya shunt inducance because of the phase shift and the resonantcharacteristics of circuits including inductance and capacity, and theresistance component of the tube input. While resonant circuits may bebroadened by shunting resistors, this solution is not practical if abroad band modulator is to CAD i less effective.

have any substantial gain and a uniform frequency characteristic.

One solution of the problem is represented in Fig. 1. A lter composed ofa capacitor I and an inductor 3 is connected between the input'terminals 5 and the input of a thermionic tube l. The output of thetube includes an impedance load 9 and terminals Il for an outputconnection. The effective and variable input reactance and resistance ofthe tube are represented respectively by the capacitor I3 and resistorI5. The characteristics fora given tube and given output load arerepresented by the curves R and X of Fig, 4. The relatively high inputreactance X and resistance R at the lower frequencies and low inputreactance and resistance at the higher frequencies correspond to anundesirable characteristic in a television modulator. The characteristicmay be modified according to this invention by connecting one terminalof a resonant circuit (including a capacitive reactor Il and aninductive reactor I9) to the grid of the tube l and the other terminalof the resonant circuit Il, I9 through a resistor 2l to the cathode ofthe tube 1. The circuit Il, I9 is adjusted to resonate at or near thehigher frequency end of the band.

The resulting operation of the modulator is as follows: At the lowerfrequencies, the circuit I1, I9 offers inductive reactance which is lowcompared to the resistance 2|, and therefore the resistance 2|terminates the lter and the input impedance of the tube may beneglected. As the .frequency of the applied currents is increased, ltheeffective impedance of the resonant circuit I9 becomes higher and theresistance 2l becomes At the resonant frequency of the circuit I9, theimpedance becomes so high that for practical purposes the inputimpedance of the tube becomes the termination for the filter I', 3. Theresultant input impedance is represented by the curve Z of Fig. 5. Thephase shift is not excessive, as shown by the lower curve of Fig. 5. Itwill be noted that the phase shift approaches 45 at the higherfrequencies and corresponds to an elementary dot of a television picturebeing displaced about one-eighth of its diameter.

In the practical application of the invention, the resonant circuit Il,I9 of Fig. 1 may be replaced by a reactor or inductor 23, as shown inFig. 2. 'I'he inductor may have a value which offers an inductivereactance throughout the frequency range, or it may be chosen so thatits distributed capacity tunes the inductor within the range. It hasbeen found advantageous t0 broad band television modulator.

- was made using three cascaded circuits according to the invention. Therst stage employed iive tubes of the type 807 which fed into an anodecircuit load consisting of a 12 ,ah inductor. The tubes of the secondstage were three type 831s. The correcting network connected to the gridof the second stage was an inductor of 12 ch and a series resistor of450 ohms. The output load of the second stage was a 17 ,ah indicator.The third stage included a single type .848 tube having an output loadincluding an inductor of 14 ,ch and an input correcting networkincluding an inductor of 34 ,uh and a series resistor ofA '700 ohms. Thefinal stage employed vtwo type 848 tubes having an input correctingnetwork including a resonant circuit consisting of Aan inductor of 5.8ch and a capacitor of 200 ,Lt/rf and a resistor of 240 ohms connected inseries therewith. In this cascade arrangement, the output load inductorsin each stage were the inductors of the filter networks corresponding toythe filter I,3 of Figs. 1, 2 and 3. The function yof the lters istobroaden the frequency re- ,.sponse.

Thus the invention has been described as a Filter networks are `usedbetween the input terminals and the amplifier tubes. Since the inputimpedance of the amplifiers varies throughout the frequency band, acorrecting network is connected to the tube input circuit so that thefilter is terminated by a resistive network at the lower frequencies andby the tube input at the higher frequencies whereby a substantiallyuniform amplication is obtained over a several-million cycle frequencyband with an increasing amplification at' the higher frequencies. Itshould be understood that the actual capacitors and resistors may bereplaced by capacity and resistance inherent in the 4amplifier tubes.

I claim as my invention:

1. A modulator for television and the like including a therrnionic tubehaving an input and an output, said input including a grid and cathodeelectrode, said output including an anode and said cathode electrode,said grid and anode reactively coupling said input and output, animpedance load connected to said output so that Vsaid input has animpedance varying with the effective impedance of said output, a lterconnected to said input, and a correcting network connected effectivelyacross said tube input, said f network including a reactor and aresistor connected in series, said reactor having a characteristie sovarying with frequency that said resistor has a negligible effect at thehigher modulation frequencies and whereby said resistor becomes theeffective termination of said filter at the lower modulationfrequencies.

2. In a device of the character of claim 1, a capacitor connected acrosssaid resistor.

3. In a device of the character of claim 1, a reactor of opposite signconnected across said 4irst-mentioned reactor whereby the reactances ofsaid reactors neutralized each other at a fre- .in series,` said reactorhaving an impedance characteristic so varying with frequency that saidresistor has a negligible eiect on the input impedance of said tube atthe higher modulation frequencies and whereby said resistor becomes theeiective input impedance of said `tube at the lower modulationfrequencies.

5. In a device of the character of' claim 4, a capacitor connectedacross said resistor.

6. In a device of the character of claim 4, a

capacitor connected across said reactor whereby its reactance isneutralized at a frequency corresponding to the higher of the applied.modulation frequencies.

'7. A modulator for television and the like including a thermionic tubehaving an input and an output, said input including a grid and cathode,said output including an anode and said f cathode, said grid and anodebeing so relatively located as to form a reactive coupling between saidinput and output, a load connected to said output whereby through saidgrid-anode reactive coupling and said output load said input hasreactive and resistive characteristics varying with frequency, and acorrecting network connected across said input, said network including aresonant circuit and a resistor connected in series, the resonantfrequency being so selected with respect to the applied modulationfrequencies that said resonant circuit offers a high impedance at thehigher modulation frequencies and at which resonant frequency saidresistor has a negligible effect and whereby said resonant circuitoffers a low impedance at the lower modulation frequencies and the saidresistor becomes the controlling input impedance.

THOMAS L. GOTTIER.

